1. Field of the Invention
The invention relates to a sulfuric acid recycle apparatus, which can be suitably applied, e.g., to a recycle apparatus for recycling sulfuric acid which is used for a wafer process in a semiconductor fabricating process.
2. Related Art
A large quantity of wafer cleaning fluid prepared by mixing sulfuric acid with hydrogen peroxide solution has been conventionally used in a semiconductor fabricating process for exfoliating resist applied to a wafer and for removing an organic matter stuck to the surface of the wafer. Upon completion of a wafer cleaning process, the wafer cleaning fluid becomes wasted fluid (hereinafter referred to as a wafer cleaning waste fluid), and the wafer cleaning waste fluid is lowered in concentration owing to water which is produced by decomposition of hydrogen peroxide solution, and hence it is difficult to be recycled. Further, since a wasting cost and a collecting cost of the wafer cleaning waste fluid are cheap, wafer cleaning waste fluid is wasted as it is.
However, considering the occurrence of today""s environmental problem which is caused by using a large quantity of such a wafer cleaning fluid and by repetitively wasting such a wafer cleaning waste fluid, a wasting process of the wafer cleaning waste fluid is not preferable in view of countermeasures against environmental protection even if a wasting cost thereof is cheap.
Further, since sulfuric acid per se is a powerful chemical of strong acidity, it cannot be wasted as it is, but need be wasted after a neutralization process and cohesion and precipitation processes are applied thereto. Under such circumstances, there is a problem that a large quantity of chemicals are required for effecting waste process of sulfuric acid.
Accordingly, it is a very important problem to reduce an amount of wasted sulfuric acid in view of countermeasures against environmental protection, and there has been desired development of techniques for concentrating sulfuric acid which is lowered in concentration upon completion of a wafer cleaning process to prepare sulfuric acid so as to be recycled for the same purpose.
It is an object of the invention to provide a sulfuric acid recycle apparatus capable of concentrating sulfuric acid, which is lowered in concentration upon completion of a wafer cleaning process, to a level to be recycled.
It is a first aspect of the invention to provide a sulfuric acid recycle apparatus for recycling sulfuric acid in a wafer cleaning fluid prepared by mixing sulfuric acid with hydrogen peroxide solution wherein the apparatus comprises a reaction bath having two openings formed of at least an introduction port and a discharge port for obtaining concentrated sulfuric acid by concentrating sulfuric acid in a wafer cleaning waste fluid introduced from the introduction port upon completion of a wafer cleaning process, then discharging the concentrated sulfuric acid from the discharge port, and a wafer processing bath for processing wafers, and a supply unit for supplying the concentrated sulfuric acid to the wafer processing bath.
That is, the sulfuric acid recycle apparatus of the invention removes residual hydrogen peroxide solution from the wafer cleaning waste fluid, or water and the like which are produced by decomposition of hydrogen peroxide solution, thereby concentrating sulfuric acid, and returning the concentrated sulfuric acid again to the wafer processing bath so as to recycle sulfuric acid in the wafer cleaning process.
The reaction bath of the invention may have a construction capable of storing the wafer cleaning waste fluid temporarily therein at the time of concentration process, and having corrosion resistance against sulfuric acid and hydrogen peroxide solution under concentration conditions. The reaction bath may be made of e.g., quarts glass.
For the supply unit for supplying concentrated sulfuric acid to the wafer processing bath, e.g., a pump serving as a unit for supplying a fluid owing to the difference in pressure may be used. In this case, although the concentrated sulfuric acid can be directly supplied from the reaction bath to the wafer processing bath, it is preferable to provide a cooling bath between the reaction bath and the wafer processing bath considering a case where the sulfuric acid is heated when it is concentrated, or a case where the concentrated sulfuric acid has a high temperature. For the pump of this type, a rotary pump, a diaphragm pump, and various other pumps may be used.
According to the invention, the sulfuric acid recycle apparatus includes the reaction bath for concentrating the sulfuric acid in the wafer cleaning waste fluid and the supply unit for supplying the thus obtained concentrated sulfuric acid to the wafer processing bath. Accordingly, water in the wafer cleaning waste fluid and hydrogen peroxide solution can be concentrated and removed, and the thus obtained concentrated sulfuric acid can be supplied to the wafer processing bath to be recycled again for use in a wafer cleaning process. Further, it is possible to continuously effect a series of processes comprising a wafer cleaning process, a concentration process of sulfuric acid in the wafer cleaning waste fluid and a recycling step of concentrated sulfuric acid, which is very efficient. As a result, a cost involving in wafer cleaning process can be reduced.
With the sulfuric acid recycle apparatus having the foregoing construction, heating units for heating the wafer cleaning waste fluid is provided in the reaction bath and a gas discharge port 14 for discharging a gas produced from the wafer cleaning waste fluid when it is heated by the heating unit, wherein a suction unit for sucking the gas is connected to the gas discharge port.
That is, although vapor is produced from water in the wafer cleaning waste fluid or the hydrogen peroxide solution when sulfuric acid is heated or concentrated, such a gas is not released to the outside owing to a natural diffusion but it is positively sucked and removed from the gas discharge port 14 formed in the reaction bath, thereby quickly concentrating sulfuric acid.
For the heating unit, it may be formed of any type if it can heat the wafer cleaning waste fluid in the reaction bath, and a heating method and the like is not limited to a particular one. That is, it may be a unit capable of heating the reaction bath from the outside or a unit capable of heating the reaction bath from the inside thereof. Particularly, if the heating unit is a type capable of directly heating the wafer cleaning waste fluid, the wafer cleaning waste fluid can be heated efficiently and concentration of sulfuric acid can be effected quickly. For such heating units, a unit having a heater made of, e.g., a tantalum and tungsten which is covered with quarts glass at the periphery thereof may be employed.
Further, for the suction unit, an aspirator and the like serving as a unit for sucking a gas utilizing decompression may be employed. If such an aspirator is employed, the reaction bath can be decompressed, thereby lowering a boiling point of water and that of hydrogen peroxide solution so as to accelerate the removal of water and hydrogen peroxide solution.
It is preferable to provide in the reaction bath a plurality of partition plates for changing an advancing direction of wafer cleaning waste fluid, and an inclination plate disposed at the portion closer to the gas discharge port rather than the partition plates for allowing concentrated sulfuric acid to flow along the surface thereof.
It is preferable to allow the wafer cleaning waste fluid to flow toward the gas discharge port provided in the reaction bath while the wafer cleaning fluid is vertically circulated by convection when a plurality of partition plates are disposed in the reaction bath. That is, when three pieces of partition plates are used, a first partition plate positioned at the introduction port side of the reaction bath is disposed to be remote from the bottom surface of the reaction bath, and a second partition plate positioned adjacent to the first partition plate is disposed to contact the bottom surface of the reaction bath, while the upper surface of the second partition plate is positioned lower than that of the first partition plate, and a third partition plate positioned at the gas discharge port side is disposed to be remote from the bottom surface of the reaction bath in the same manner as the first partition plate.
With the disposition of the partition plates as set forth above, the wafer cleaning waste fluid introduced from the introduction port moves from a gap defined between the lower surface of the first partition plate and the bottom surface of the reaction bath to the second partition plate, and thus moved wafer cleaning waste fluid moves to the third partition plate when it is filled up at the height exceeding the upper surface of the second partition plate, and it stays between the second partition plate and the inclination plate. Further, when concentrated sulfuric acid between the third partition plate and the inclination plate stays therebetween at the height exceeding the upper surface thereof, it flows along the surface of the inclination plate and moves to the gas discharge port. Accordingly, the wafer cleaning waste fluid can be steadily heated so that it can be sufficiently concentrated.
Further, since the inclination plate is provided in the reaction bath so as to allow concentrated sulfuric acid to flow along the surface thereof and drop in the reaction bath, a surface area of concentrated sulfuric acid becomes large when it is supplied to the gas discharge port, while removal of water and the like which slightly remain on the surface of the inclination plate 16 is further accelerated, thereby improving a quality of concentrated sulfuric acid. It is preferable that the inclination plate has irregularities thereon so as to enhance concentration of concentrated sulfuric acid and improve a quality thereof. When the irregularities are formed on the surface of the inclination plate, a surface area of the inclination plate becomes larger, thereby efficiently removing water and the like. For a material of the partition plates and the inclination plate, a material having corrosion resistance against strong sulfuric acid at high temperature can be used, e. g., quarts glass used in the reaction bath can be also employed.
Further, it is preferable that the reaction bath has a waterdrop storage bath capable of preventing waterdrop stuck to a ceiling surface of the reaction bath from being mixed with concentrated sulfuric acid. It is preferable that the waterdrop storage bath is disposed over the gas discharge port so as to prevent water from being mixed with finally processed concentrated sulfuric acid. Further, it is ideal that the waterdrop storage bath is disposed to cover the lower portion of the gas discharge port if the gas discharge port has been already formed in the reaction bath. With the provision of the waterdrop storage bath, it is possible to prevent the concentration of concentrated sulfuric acid from lowering, which is caused by the mixture of water, which is once removed from the wafer cleaning waste fluid, with concentrated sulfuric acid.
It is preferable to supply fresh sulfuric acid to concentrated sulfuric acid which is obtained by heating and concentrating wafer cleaning waste fluid when sulfuric acid of concentrated sulfuric acid is reduced. For means for supplying fresh sulfuric acid, it is possible to employ means for pumping up fresh sulfuric acid from a sulfuric acid tank, which is installed separately in the sulfuric acid recycle apparatus, by a pump like the supply unit, thereby supplying fresh sulfuric acid to the concentrated sulfuric acid. With the provision of the supply means or a supply unit capable of supplying fresh sulfuric acid, if concentration of concentrated sulfuric acid obtained from the wafer cleaning waste fluid is lower than a desired value, concentration thereof can be made higher. Even with the employment of such a supply unit, the amount of use of fresh sulfuric acid can be remarkably reduced compared with a case where all the wafer cleaning waste fluid are wasted.
It is preferable that the reaction bath 10 has a heating temperature therein ranging from 150xc2x0 C. to 315xc2x0 C., more preferably 180xc2x0 C. to 250xc2x0 C. If the heating temperature exceeds 315xc2x0 C., there occurs drawbacks that parts used for the reaction bath 10 are limited to a high fire-resistive material, having a high fire resistance, and also since the amount of vapor produced in the reaction bath 10 increases, the sulfuric acid recycle apparatus becomes large-sized and complex. On the other hand, if the heating temperature is less than 150xc2x0 C., efficiency of concentration of sulfuric acid (dehydration efficiency) becomes worse so that an expected effect can not be achieved.